1. Field of the Invention
The present invention relates to a method of forming a plurality of quantum holes of nanometer scale, a light-emitting device utilizing quantum dots, which is able to emit light of a predetermined color, and its fabricating method.
2. Description of the Prior Art
There are various lamps which convert electrical energy into electromagnetic radiation at visible wavelengths, that is, electric lamps such as incandescent lamps, halogen lamps, etc. Generally, such electric lamps can radiate monochromatic light only. Accordingly, they require various color filters and gels for the emission of light with various colors. However, such a radiation system as depends on many filters and gels for the expression of various colors of light has the significant drawback of a need to substitute filters or gels of new or different colors or exchange them with pre-existing ones whenever new or different colors are needed. Not only is the substitution or exchange inconvenient, but also it is impossible to express various colors precisely through color filters or gels.
Therefore, there was a need to develop new technology by which various colors of light can be expressed precisely and conveniently. Complying with such a need, light emitting diodes, which take advantage of semiconductors in emitting light of desired colors, have been developed and extensively used.
Representative of the light emitting diodes is a bulk-type light emitting diode, which comprises a PN junction layer between a P-type semiconductor and an N-type semiconductor, each of which has an electrode. When an electric field is applied across the electrodes, holes of the P-type semiconductor and the electrons of the N-type semiconductor move toward the PN junction layer and are combined with each other thereat, excited and transited, emitting the light corresponding to the energy difference.
In order to better understand the background of the invention, a conventional bulk type light emitting diode will be explained in conjunction with the accompanying drawings.
Referring to FIG. 1, there are shown structures of a conventional bulk type light emitting diode. As seen in FIG. 1, the conventional bulk type light emitting diode consists of a P-type semiconductor layer 12 and an N-type semiconductor layer 14 with a PN junction layer 16 therebetween. On the P-type semiconductor layer 12 consisting of GaN, INGaN is grown in crystal bulk to form the PN junction layer 16 atop which the N-type semiconductor layer 14 is then formed. Each of the P-type semiconductor layer 12 and the N-type semiconductor layer 14 may be composed of a plurality of layers.
The expression of a desired color of light is accomplished by a combination of three primary colors of light. In order to emit light of a desired color, thus, there are needed three bulk type light emitting diodes 10, 10a and 10b which can radiate at red, green and blue wavelengths, respectively.
The three light emitting diodes which radiate wavelengths corresponding to red, green and blue colors, respectively, differ from one another in the composition of the PN junction layer 16, particularly, the Indium(In) portion of the InGaN composition. That is, when PN junction layers are formed of single crystals of InGaN, they are grown with different Indium (In) compositions suitable for use in the emission of red, green and blue light, respectively. The reason why different Indium (In) compositions are used is that indium (In) is used to regulate the recombination energy between the carriers of electrons and holes.
Bulk type light emitting diodes 10, 10a and 10b, which emit light of three primary colors, are cut into individual light emitting diode elements of red 18, green 18a and blue 18b. Such light emitting diode elements are used individually or in a combined manner on a display panel.
With reference to FIG. 2, there is a structure of a display panel on which conventional bulk type light emitting diodes are employed, in combination, to express certain images. As seen in this figure, a plurality of the bulk type light emitting diodes 10, 10a and 10b are combined to form a display panel. For example, the display panel 20 is composed of 13 red light emitting diodes 10, 8 green light emitting diodes 10a, and 4 blue light emitting diodes 10b. 
With reference to FIG. 3, there is a curve showing the light intensity versus wavelength in the display panel of FIG. 2. As recognized from the curve, the display panel 20 consisting of conventional bulk type light emitting diodes 10, 10a and 10b has a large distribution of wavelengths when the light intensity is weak. The large distribution of wavelengths is also found where the light intensity of the display panel is strong. That is, the wavelengths of the light emitted from the display panel 20 are distributed in a wide range even at the peak of the curve.
Accordingly, the display panel 20 consisting of a plurality of conventional bulk type light emitting diodes 10, 10a and 10b can expresses a color only vaguely, with a broad range of wavelengths around the wavelength pertinent to the color, but not correctly with the precisely pertinent wavelength. That is, the conventional display panel 20 generates radiation at a wide range of wavelengths of light to express a color, so that viewers can only recognize a color not identical, but similar to the intended color due to limitations of human vision. Consequently, it is impossible for the conventional display to display a color of light entirely at the wavelength intrinsic to the color.
Besides, because of its large size, such a conventional bulk type light emitting diode is difficult to apply to a small size display device which is capable of expressing various colors and delicate images.
It is, therefore, an object of the present invention to provide a method of forming quantum holes of nanometer scale on semiconductor substrates.
To form such quantum holes, an ion beam scanner is utilized in the present invention. In the ion beam scanner, ions are projected from an ion gun onto a semiconductor substrate. During the projection, ions are focused into an ion beam whose focal point is controlled to determine the diameter of the ion beam, and the ion beam is accelerated. When being incident upon the semiconductor substrate, the ion beam is deflected so as to form a plurality of quantum holes.
It is another object of the present invention to provide a light-emitting device with quantum dots, capable of expressing colors of light clearly.
It is a further object of the present invention to provide a method for fabricating such a light-emitting device.
The light-emitting device can be fabricated by growing an intrinsic semiconductor layer on a P-type semiconductor layer, forming a plurality of quantum holes on the grown intrinsic semiconductor layer, filling the quantum holes with a material smaller in energy band gap than the intrinsic semiconductor by single crystal growth, and overlaying an N-type semiconductor layer on the quantum hole layer.
Advantageously, the light-emitting devices can be cut into unit light emitting elements of micron sizes. Also, the semiconductor can selectively emit light of the three primary colors according to the materials by which single crystal are grown within the quantum holes. When being integrated, the unit light emitting elements can find various applications in the image display industry, such as illumination apparatuses, electric signs, and advertising panels.
In addition, the illumination using the light emitting elements of the present invention can be varied in color and intensity under digital control. Further, the light emitting elements of the present invention make image display free from size limitation. For instance, a matrix on which unit light emitting elements capable of emitting red, green and blue light separately are integrated can be applied to digital illumination in which predetermined colors are expressed under digital control. Therefore, the present invention can make a contribution to the development of illumination technology from the simplest level, e.g., simply brightening; to more complex levels, e.g., varying the color and intensity of light according to the rhythm and tone of music.
In association with digital technology, the light emitting elements of the present invention can express images very clearly and regulate colors precisely. In addition to generating neither heat nor infrared light, the light emitting elements of the present invention are almost semi-permanent. Since the three primary colors of light can be expressed clearly using only three unit light emitting elements, they can reduce sizes of displays, such as electric display panels, as well as enabling the displays to express clear images. Wavelengths of the light emitted from the semiconductor for use in a light emitting device of the present invention are distributed in a narrow range with the wavelength distribution curve having a sharp peak, so that complete images can be achieved on screens which use the semiconductor of the present invention.
Another feature of the present invention resides in the representation of bright images because the unit light emitting elements of the present invention are of micron sizes and can express pure colors and increase the intensity of the light emitted. In aspects of the intensity of light energy and the coherence of light wavelengths for a color, the unit light emitting elements of the present invention are closer to laser light emitting devices than to currently commercialized bulk type light emitting diodes. Further, the semiconductor allows the fabrication of a light emitting device which is able to emit radiation in a highly coherent range of wavelengths intrinsic to an intended color.